The Fermi Paradox: If Aliens Exist, Why Is Nobody Talking to Us?

In 1950, physicist Enrico Fermi sat down to lunch with colleagues at Los Alamos National Laboratory and asked a question that has not been satisfactorily answered in the 75 years since. Where is everybody? That’s it. That’s the whole question. And it turns out to be one of the most unsettling things anyone has ever said at lunch.

The Setup

The universe is approximately 13.8 billion years old. It contains somewhere between 200 billion and two trillion galaxies. Our own galaxy, the Milky Way, holds an estimated 100 to 400 billion stars. A significant portion of those stars have planets. Scientists currently estimate there are around 40 billion Earth-like planets in the Milky Way alone.

Given those numbers, the statistical probability of Earth being the only planet that ever developed life seems, to put it generously, unlikely. Even if intelligent life is extraordinarily rare, the sheer scale of the universe suggests it should have happened somewhere else. Somewhere else many times over. Long before Earth existed.

The Milky Way is old enough that a civilization arising just a few million years before us would have had time to colonize the entire galaxy by now, even using slow sub-light travel. Furthermore, they would have had billions of years to send signals or make their presence known in some way.

And yet we have found nothing. No signals have reached us. No visitors have arrived. No evidence of anyone else exists, anywhere, ever. That gap between expectation and observation is the Fermi Paradox.

The Drake Equation and Why It Makes Things Worse

Before getting into the proposed solutions, it helps to understand the Drake Equation, which astronomer Frank Drake developed in 1961 to estimate how many communicating civilizations might exist in the Milky Way right now.

The equation multiplies together a series of factors: the rate of star formation, the fraction of stars with planets, the fraction capable of supporting life, the fraction where life actually develops, the fraction where intelligent life develops, the fraction that develop detectable communication technology, and finally how long those civilizations survive.

The first few factors are now reasonably well-estimated by astronomers. The last few are complete guesses. Depending on how optimistic you are about those later variables, the equation produces answers ranging from tens of thousands of active civilizations right now to essentially zero.

A single equation producing both results depending on your assumptions is itself a kind of answer. We do not actually know how rare we are. We just know we seem to be alone, and that this seeming-aloneness is strange given the size of the stage.

The Proposed Solutions, Ranked by How Much They Bother Me

There are many proposed explanations for why the universe appears empty. None of them are fully satisfying, which is arguably the point.

The simplest explanation is that we are alone. Life, particularly intelligent life, may be so extraordinarily rare that Earth genuinely is the only place it has developed in the observable universe. Scientists call this the Rare Earth hypothesis. It is technically possible. Sitting with the scale of what we would have to be the only example of, however, is deeply strange.

The Great Filter

The Great Filter is a more unsettling version of the same idea. Economist Robin Hanson proposed the theory in 1998. It suggests that somewhere in the progression from basic chemistry to spacefaring civilization, there is a filter so difficult to cross that almost nothing makes it through.

If the filter is behind us, something like the emergence of complex cells or multicellular life, then we got lucky and most life never makes it as far as we have. If the filter is ahead of us, then almost every civilization that reaches our level eventually destroys itself before spreading to other star systems. That second option is the scenario that should make you put your phone down for a second.

Additionally, there is what people call the Dark Forest theory, popularized by Chinese science fiction author Liu Cixin. The idea is that the universe is full of civilizations, but all of them are hiding. Any civilization that reveals itself risks destruction by another that cannot afford the risk of a potential competitor. Under this theory, silence is not evidence of emptiness. Rather, it is evidence of everyone being terrified of everyone else. Sending out radio signals, in this framework, is roughly equivalent to firing a flare in a dark forest full of unknown predators.

The Zoo hypothesis suggests advanced civilizations are aware of us and have deliberately chosen not to make contact, either to observe us without interference or because we have not yet reached the point that makes contact worthwhile. This one is frustrating in a specific way because it cannot be disproven. Finally, some researchers point to the Simulation hypothesis: if our universe is a simulation, the absence of alien contact might simply be a resource-saving decision by whoever is running it.

What We Have Actually Tried

SETI, the Search for Extraterrestrial Intelligence, has scanned the sky for radio signals since 1960. In that time, researchers found one genuinely unexplained signal: the Wow! signal, recorded on August 15, 1977, by astronomer Jerry Ehman at Ohio State University.

The signal was strong, narrowband, and matched almost exactly what a transmission from deep space would look like. It lasted 72 seconds and came from the direction of the constellation Sagittarius. Ehman circled it on the printout and wrote “Wow!” in the margin, which is how it got its name. Nobody has detected it again. Researchers have proposed various natural sources and largely rejected them all. Today it remains the best candidate for extraterrestrial contact in the history of the search, which is either exciting or extremely anticlimactic depending on your expectations.

In 2015, the Breakthrough Listen initiative launched with a ten-year, $100 million commitment to search for extraterrestrial intelligence. As of the time of writing, it has found nothing definitive. In 2017, a strange elongated object called Oumuamua passed through the solar system on a trajectory inconsistent with any known comet or asteroid. Harvard astrophysicist Avi Loeb publicly suggested it might be alien technology. Most of his colleagues disagreed. The debate continues, and Oumuamua is long gone.